Shuttle-type-printers and methods for operating same

Abstract: A printing system (10) for a shuttle-type printer includes a platen (12) and a carriage (22) configured to move bidirectionally across the platen. An optically responsive demarcation (70) is provided on the platen outside of the media feed path (19). A printhead (40) and an optical sensor (50) are disposed on the carriage (22). During operation, the carriage is operable to position the optical sensor (50) over the platen demarcation (70), whereby the optical sensor generates a position signal when it detects the platen demarcation. A control subsystem (60) is operably coupled to the optical sensor (50) to determine absolute position of the carriage relative to the platen (12) in response to optical identification of the platen demarcation (70) by the optical sensor (50). Several methods for operating such a printing system are also described.

Description: Technical Field This invention relates to shuttle-type printers and methods for operating them. Background of the Invention Shuttle-type printers are a class of printers having a movable shuttle or carriage that traverses back and forth across a printing surface. A printhead is mounted on the shuttle and synchronized with shuttle movement to print desired images. The shuttle class of printers includes both impact printers, such as dot matrix and daisy-wheel printers, and non-impact printers, such as ink-jet printers. A shuttle drive mechanism maneuvers the shuttle over the printing surface. The shuttle drive mechanism typically consists of a motor, and a belt and pulley assembly which operably couples the shuttle to the motor. Common motors used in such mechanisms include a DC motor which changes speed and direction in relation to the level and polarity of DC voltage applied thereto, and a stepper motor which changes speed and direction in response to intermittent pulses. The stepper motor is less effective at providing precise position control as compared to the DC motor plus shaft encoder; but, the stepper motor is advantageously less expensive than the DC motor and encoder. One problem that plagues shuttle-type printers is the inherent lack of precise positional control due to mechanical tolerances of the shuttle drive mechanism. The motor and drive belt assembly possess manufacturing variances that induce slight, but acceptable, errors in the shuttle positioning process. These errors are manifest in assembled printers and vary from printer to printer. Accordingly, it would be advantageous to identify the inherent mechanical errors within an assembled printer and compensate for them. JP-A-62226765 describes how to accurately set a home position of a printer carriage by providing a reading mark on the printer platen which is detected by a home position reading means. This invention, as specified in the claims hereinafter, overcomes the above drawbacks by providing a low cost, automated system and associated operating methods for determining absolute carriage position relative to the platen. In contrast to JP-A-62226765, the present invention employs an aperture in the platen for determining the carriage home position. Disclosure of the Invention According to one aspect of this invention, a printing system for a shuttle-type printer includes a platen and a carriage mounted adjacent to, but spaced from, the platen to permit passage of a recording media therebetween. The media flows along a media feed path having a width effective to cover a first portion of the platen while leaving exposed a second portion of the platen. The carriage is configured to move bidirectionally across the platen to be positionable (1) over the first portion of the platen associated with the media path, and (2) over the second portion of the platen outside of the media path. An optically responsive demarcation in the form of an aperture is provided in the second portion of the platen outside of the media path. The printing system also includes a printhead disposed on the carriage to form printed images on the recording media. An optical sensor is also disposed on the carriage, whereby the optical sensor has a light source oriented to emit a light beam toward the platen and a light sensitive detector aligned to detect reflected light. The carriage is operable to position the optical sensor over the platen demarcation, whereby the optical sensor generates a position signal when it detects the platen demarcation. From this signal, a control subsystem determines position of the carriage relative to the platen. Brief Description of the Drawings Preferred embodiments of the invention are described below with reference to the following accompanying drawings depicting examples embodying the best mode for practicing the invention. Fig. 1 is a diagrammatic illustration of a printing system for a shuttle-type printer according to this invention, and Fig. 2 is a drawing used to demonstrate a method for determining carriage position. Detailed Description of the Preferred Embodiments Fig. 1 shows a printing system 10 of a shuttle-type printer. System 10 includes a platen 12, a shuttle assembly 20, a printhead 40, an optical sensor 50, and a control subsystem 60. Platen 12 is preferably stationary and supports a recording media 14 during printing. Recording media 14 has an upper edge 15, a first side edge 16, and a second side edge 18. Media 14 may be a continuous form or individual sheet stock, and it can consist of paper, adhesive-backed labels, or other types of printable matter. A media feed mechanism (not shown), such as friction rollers or a tractor feed system, is used to drive the media through the printer along a media feed path. The media feed path is represented by dashed boundary lines 19 and has a width effective to coincide with a first portion of platen 12 while leaving exposed a second portion of the platen. More specifically, platen 12 has a center region 17 that defines media feed path 19 and two opposing end regions 21, 23 that extend beyond the media feed path. Shuttle assembly 20 includes a carriage 22 slidably mounted on a fixed, elongated rod 24 to move bidirectionally across the platen 12. Carriage 22 preferably maneuvers over the full width of the platen to be positionable over the media feed path 19 at the platen center region 17 and over the two opposing end regions 21, 23 outside of media feed path 19. Carriage 22 has a nose section 25 that is adjacent to, but spaced from, the platen 12 to permit passage of the recording media 14 therebetween. Shuttle assembly 20 further includes a drive subassembly 26 that is mechanically coupled to drive carriage 22 back and forth along rod 24. Drive subassembly 26 includes a wire or belt 28 attached to carriage 22 and wound around opposing pulleys 30, and a motor 32 connected to power one of the pulleys. Preferably, motor 32 is a stepper motor, but a DC motor can also be used. A rotary encoder 34 is coupled to the motor drive shaft to monitor incremental shaft rotation. This incremental count provides feedback data for use in positioning and controlling the carriage. The shuttle assembly 20 is illustrated in one typical form for explanation purposes and its construction is well known in the art. However, other types of shuttle assembly configurations may be employed in this invention. Printhead 40 is mounted on nose section 25 of carriage 22 in juxtaposition with platen 12. Printhead 40 is diagrammatically represented as a block on nose section 25 of carriage 22 and can be embodied as an ink-jet printhead, a dot matrix printhead, a daisy-wheel, or any other type of printhead carried on a shuttle. An optical sensor 50 is also mounted on carriage 22 to be positionable above platen 12 and/or media 14. Optical sensor 50 includes a light source (e.g., photoemitter, LED, laser diode, super luminescent diode, fiber optic source) oriented to emit a light beam toward platen 12 and a light sensitive detector (e.g., photodetector, charged couple device, photodiode) aligned to detect light reflected from the platen or media. Optical sensor 50 is preferably mounted adjacent to, and in substantial alignment with, the printhead 40 to monitor lines of text or other images that have already been printed. The control subsystem 60 of printing system 10 consists of various components used to monitor and control operation of the printing system. It includes a printhead controller 62, an optical sensor controller 64, a carriage controller 66, a memory 68, and a processor 69. These components are illustrated in block form for clarity of discussion. Printhead controller 62 is electrically coupled to printhead 40 to manage the tasks associated with transforming digital data downloaded to the printer into desired patterns to be applied on the recording media. Optical sensor controller 64 is electrically coupled to monitor signals generated by optical sensor 50. Carriage controller 66 is configured to manage motor 32 and receive incremental motion feedback from rotary encoder 34 to controllably position carriage 22 at selected locations relative to platen 12 or media 14. Memory 68 is preferably a non-volatile, randomly accessible memory which stores position-related information. In practice, control subsystem 60 is embodied as one or more microprocessors, microcontrollers, ASICs, or other circuitry and logic. Printing system 10 also has at least one optically responsive platen demarcation 70 provided at one end 21 of platen 12. Preferably, a platen demarcation is provided at each of the two opposing end regions 21 and 23 outside of media feed path 19, as shown by demarcations 70 and 72, respectively. In this manner, when media 14 is fed through printing system 10 between carriage 22 and platen 12, the demarcations 70 and 72 remain exposed beside the media. The demarcations possess a distinctly different optical density as compared to that of the platen to induce a detectable change in signal output when the optical sensor 50 passes over the demarcation. The demarcations are embodied as apertures formed in the platen. The demarcations 70, 72 are used in conjunction with optical sensor 50 to enable measurement of absolute carriage position relative to platen 12, as will be described below in more detail. Carriage Position Control The printing system 10 is capable of conducting many diverse tasks. One task of this invention involves determining absolute carriage position relative to the platen. Carriage 22 is moved to platen end region 21 beyond the media feed path 19 to align optical sensor 50 with optically responsive platen demarcation 70. When optical sensor 50 overlies demarcation 70, the emitted light beam passes partially through the aperture resulting in less reflectance. This yields a detectable transition in light reflectance from platen 12 to aperture 70, causing a variation in the signal output from optical sensor 50. In other words, the optical sensor generates a position signal (i.e., a change in signal level) when it detects platen demarcation 70. Upon receipt of the position signal, the control subsystem 60 can monitor the carriage position via carriage controller 66 and determine an absolute position of carriage 22 relative to platen 12. Another technique according to this invention involves identifying the inherent mechanical-induced position errors of the printing system and then compensating for them. From its position over the first platen demarcation 70, the carriage 22 is moved away from the demarcation 70 across the platen 12 and beyond the media feed path 19 to the opposing end region 23. The carriage movement is halted when the optical sensor 50 is aligned with and detects second optically responsive platen demarcation 72. Upon detection, the reflectance level changes and the optical sensor 50 generates a second position signal. As the carriage 22 traverses the platen, a rotary encoder 34 outputs pulses for each incremental step. The pulses are fed to carriage controller 66 and conveyed to processor 69. The processor counts the pulses to measure a displacement distance traveled by the carriage 22 from its initial position above platen demarcation 70 to its final position above demarcation 72. Processor 69 can then compare the displacement distance to an ideal distance value stored in memory 68 to derive a carriage position error. As an example of this method, assume that the platen demarcations 70 and 72 are 22.86 cm (nine inches) apart and the printer is configured to print 118.11 dots per cm (300 dots per inch (dpi)). The ideal count stored in memory is 2700 steps (i.e., 22.86 cm x 118.11 incremental steps/cm (9 inches x 300 incremental steps/inch) = 2700 steps). However, if the encoder returns an actual displacement distance of 2695 steps, the printing system has an inherent error of 5 steps which equates to a carriage position error of 0.042 cm (1/60th inch) for the 22.86 cm (nine inch) range. The carriage position error is most likely a result of imprecise mechanical aspects inherent in the carriage assembly 20. Because the demarcations 70 and 72 provide a fixed scale which is known by control subsystem 60, the position performance of carriage assembly 20 can be isolated and evaluated for inherent error. The mechanically-induced error is likely to remain approximately constant throughout the prescribed life of the printer. Accordingly, once this error is measured, the printing system 10 can be adjusted to compensate for it. Alternatively, some errors become manifest over time due to mechanical wear and the like. Using the unique techniques described herein, the printer can periodically measure the errors and dynamically alter operating parameters to correct for the errors. Detecting and adjusting for tolerance error is explained in more detail with reference to Fig. 2. This example assumes the above error of 5 incremental steps 0.042 cm (1/60th inch) over a 22.86 cm (nine inch) range. An arbitrary position over the recording media is selected by the printer. The carriage is initially positioned over the left-side platen demarcation 70 and then moved to the arbitrary position. Control subsystem 60 monitors the distance traveled during the rightward pass and measures a rightward pass RP count of, say, 1753 steps. The carriage is then moved to the right-side platen demarcation 72 to initiate a leftward pass back toward the arbitrary position. For this operation, the leftward pass LP count is, say, 942 steps. The sum of the two passes yields a total count of 2695, which reflects the presumed error of 5 steps. Now assume the printer is adjusted to compensate for the inherent 0.042 cm (1/60th inch) error (for the 22.86 cm (nine inch) range). The location of the arbitrary position relative to the demarcations is known by the processor 69. If the arbitrary position is ideally located at the 1756th step from the left-side demarcation, the control subsystem would output position control information indicative of a slightly lower value, such as 1753 steps, to correct the mechanical error in the carriage assembly 20. Corrected values for negating the effects of the position error can be computed in a variety of ways. One technique, used in the above example, is to derive a corrected value which is proportional to the distance across the platen. For instance, to accommodate for a -5 step error in a 2700 step range, the control subsystem subtracts one step for every 540 steps made by the carriage across the platen. Another technique is to fully correct for the entire 5 step error each time the carriage changes direction. This would compensate for errors induced by, for example, excessive slack in the belt 28. The system of this invention is advantageous because it provides a low cost solution to mechanical error inherent in carriage assemblies. The system is well suited for low cost printers which employ less precise stepper motors, as the unique control process yields higher precision results comparable to those obtained by more expensive printers.

Claims: A printing system for a shuttle-type printer, comprising: a platen (12); a carriage (22) adjacent to, but spaced from, the platen (12) to permit passage of a recording media (14) therebetween along a media feed path (19), the media feed path having a width effective to cover a first portion of the platen while leaving exposed a second portion of the platen; the carriage (22) being configured to move bidirectionally across the platen (12) to be positionable (a) over the first portion of the platen associated with the media path (19), and (b) over the second portion of the platen outside of the media path; a printhead (40) disposed on the carriage (22) to form printed images; an optically responsive platen demarcation (70) formed as an aperture in the second portion of the platen (12) outside of the media path (19); an optical sensor (50) disposed on the carriage (12), the optical sensor (50) having a light source oriented to emit a light beam toward the platen and a light sensitive detector aligned to detect reflected light, the optical sensor (50) generating a position signal when the platen demarcation (70) is detected; and a control subsystem (60) operably coupled to the optical sensor (50) to determine position of the carriage (22) relative to the platen (12) in response to optical identification of the platen demarcation (70) by the optical sensor (50). A printing system according to claim 1 wherein: the platen (12) has a center region (17) and two opposing end regions (21, 23), the center region defining the first portion of the platen and the end regions defining the second portion of the platen; the printing system further comprises: an optically responsive platen demarcation (70, 72) formed as an aperture in the platen at each of the two opposing end regions (21, 23), the carriage (22) being operable to position the optical sensor (50) sequentially over a first platen demarcation (70) at one end region (21) of the platen and then over a second platen demarcation (72) at the other end region (23) of the platen; and a monitor (34) for measuring the distance traveled by the carriage (22) from the first demarcation (70) to the second demarcation (72). A method of operating a shuttle-type printer, the shuttle-type printer having a platen (12) with one or more optically responsive demarcations (70, 72) each formed as an aperture in the platen, a carriage (22) which moves bidirectionally across the platen, and a printhead (40) and an optical sensor (50) mounted on the carriage, the method comprising the following steps: moving the carriage (22) in a direction across the platen (12) and until the optical sensor (50) detects a first optically responsive demarcation (70) on the platen; generating a first position signal when the first platen demarcation (70) is optically detected; and determining an initial position of the carriage (22) relative to the platen (12) in response to the first position signal. A method according to claim 3 comprising the following additional steps: moving the carriage (22) in a direction away from the first platen demarcation (70) across the platen (12) and until the optical sensor (50) detects a second optically responsive demarcation (72) on the platen; generating a second position signal indicative of a final position of the carriage (22) relative to the platen in response to optically detecting the second platen demarcation (72); and measuring a displacement distance traveled by the carriage (22) from the initial position to the final position. A method according to claim 4 comprising the following additional steps: providing an ideal displacement distance between the first and second demarcations (70, 72) on the platen (12); comparing the measured displacement distance with the ideal displacement distance; deriving an error when the measured displacement distance is not identical to the ideal displacement distance; and compensating for discrepancy between the measured and ideal displacement distances in response to the error. A method according to claim 3 comprising the following additional steps: feeding a recording media (14) between the platen (12) and carriage (22) along a media path (19) in a manner that leaves the first optically responsive platen demarcation (70) exposed beside the recording media (14); and moving the carriage (22) beyond the recording media and until the optical sensor (50) detects the first platen demarcation (70).

Inventors: KHORMAEE IZADPOUR, KHORMAEE, IZADPOUR
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1994
Main classification: B41J19
IPC codes: B41J19, B41J29, B41J25, B41J11